I don't remember for sure, but my memory is that manpower centers are like other factories. Nothing is produced as long as they are over 50% damaged. Once they are under 50% damaged they check to see if they produce, with the percent chance of producing equal to 100-damage. For manpower centers, manpower is what is being produced. Most cities that have been captured and occupied will have lost manpower centers, but whatever is left, will eventually start producing manpower again.